In general, a laser device for generating a laser light for precise machining has a structure, such as shown in FIG. 13, which includes a discharge tube 1, electrodes 21 and 22, a high frequency power source circuit 4, a total reflection mirror 71, and a half-mirror 72. A high frequency discharge is generated in a gas 6, such as He, Ne, CO.sub.2 and the like, flowing through a discharge tube 1, and accordingly, a laser light oscillation is generated. The characteristics of the voltage between electrodes V.sub.1 and the laser output W, with regard to the electric current I between electrodes, are shown in FIG. 14. Namely, in accordance with an increase of the current I, the voltage rises from 0 to a point a' corresponding to the discharge starting voltage V.sub.s ' passing through a point e'. Then, when the discharge is started, the voltage suddenly drops to a point b', and subsequently changes along a curve b'-c'. Following the decrease of the current I, the voltage changes along a curve c'-b', passing through point b', and when the voltage reaches point a', rises suddenly to point e' and then falls to point 0.
In correspondence with this change of the voltage V, the characteristics of the laser output W as shown in the lower side of FIG. 14 are obtained. As shown in this characteristic curve of the laser output W, during the reduction of the current I, the laser output becomes extinct at a point d'. Namely, in the characteristic curve of the laser output W, within the range between points d' and e', the laser output can not be controlled by the current I. Therefore, a problem arises in that it is very difficult to obtain a stable control of a laser output when the laser output is low, and thus it is impossible to meet the requirements for a precise machining operation.